Ok, I filled in the gaps with mud. Sanded and laid fiber glass stip over the seam between the tip and aluminum fin. Sanded again. Finally filled in with mud and sanded smooth.
Some say the dis-similar materials will cause the seam to eventuall crack open. I don't think so for two reasons: 1) I've seen lots of RVs that do this and I've never seen a crack and 2) the amount of length is so short that I don't think there will be siginificant dis-similar expansion.